Ecology: Communitiy

• Community: an assemblage of populations of various species living close enough for potential interaction

Ecological Niches and Natural Selection

• Ecological niche: the total of a species’ use of biotic and abiotic resources
  • an ecological niche can also be thought of as an organism’s ecological role
• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches
• Resource partitioning: differentiation of ecological niches, enabling similar species to coexist in a community

Niches

• Fundamental niche: the niche potentially occupied by that species
• Realized niche: the niche actually occupied by that species
• As a result of competition, a species’ fundamental niche may differ from its realized niche

Types of Community Interactions

• Interspecific interactions: relationships between species in a community
  • Competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation
• Interspecific interactions can affect the survival and reproduction of each species,
  • positive (+), negative (–), or no effect (0)

Competition

• Intraspecific competition: competition between individuals of the same species
• Interspecific competition: competition between individuals of different species
• Exploitation competition: organisms compete indirectly through the consumption of a limited resource
• Interference competition: individuals interact directly with one another by physical force or intimidation
• Strong competition can lead to competitive exclusion, local elimination of a competing species
  • The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Predation

• Predation (+/– interaction): interaction where one species, the predator, kills and eats the other, the prey
  • Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison
  • Prey display various defensive adaptations
  • Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls
  • Animals also have morphological and physiological defense adaptations
  • Cryptic coloration, or camouflage, makes prey difficult to spot

Antipredator Strategies

• Chemical defense 
  • ex: Bombardier beetle ejects hot spray
• Displays of intimidation: deceive predator about ease of eating prey
  • ex: Porcupine fish inflates itself
• Importance of predation depends on whether it’s a donor-controlled system or a predator-controlled system
  • Donor-controlled system: prey supply is determined by factors other than predation, such as food supply
  • Removal of predators has no effect on prey density
  • Predator-controlled system: action of predator feeding reduces supply of prey
  • Removal of predator results in increase in prey

Plants vs Herbivores

• Herbivory can be lethal to small species
• Usually not lethal to larger species
• Generalist herbivores: can feed on many plant species
• Specialist herbivores: restricted to one or two host plants
• Two proposals explain why every plant isn’t consumed
  • Predators and parasites keep herbivore numbers low
  • Plant defenses make a difference

Plants Defenses

• Array of unusual and powerful chemicals

  • Secondary metabolites: not part of primary \n energy-generating metabolic pathway
  • Alkaloids(nicotine in tobacco, morphine in poppies, cocaine in coca, and caffeine in coffee)
  • Phenolics (lignin in wood and tannin in leaves)
  • Terpenoids (in peppermint)

• Mechanical defenses like thorns and spines

• Herbivores can overcome plant defenses

  • Detoxify using two pathways
  • Oxidation: catalysis of secondary metabolite to corresponding alcohol by mixed-function oxidases (MFOs)
  • Conjugation: unites results of oxidation with another molecule to create inactive and readily excreted product

• Parasitism (+/– interaction): one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process (does not usually kill it outright).

  • Endoparasites: parasites that live within the body of their host
  • Ectoparasites: parasites that live on the external surface of a host

Mutualism

• Mutualism: close associations between species in which both species benefit
• Trophic mutualism: both species utilize a common resource
  • Example: Leaf-cutting ants and fungus
  • Ants chew up leaves to feed to fungus they care for
  • Fungus produces gongylidia as ant food
  • Ants circumvent chemical defenses of leaves
• Defensive mutualism: animal defends a plant or herbivore
  • ex: Ants protect aphid, aphid secretes honeydew
  • Facultative mutualism: the species can’t live apart
  • ex: Ants nesting in acacia trees defend the tree and trim away competing plants
  • Obligatory mutualism: neither species can live alone
• Dispersive mutualism: one species receives food in exchange for helping the flower spread their pollen
  • Plant would like pollinator with fidelity to one species that moves quickly among individuals
  • Animal wants to be a generalist to get the most food in a small area, reducing energy expenditure
  • Mutualisms are beneficial – but optimal needs of each party can be different

Commensalism

• Commensalism: one member derives a benefit while the other is not benefited or harmed
• ex: Epiphytes growing in trees do not harm the trees
• ex: Cattle egrets benefit from cattle stirring up insects
• Phoresy: one organism uses another for transportation
  • Flower-inhabiting mites use hummingbird nostrils
• Cheating
  • Grass-pink orchid produces no nectar, but it mimics the nectar-producing rose pogonia and is therefore still visited by bees
  • Plants cheat seed-dispersal agent out of meal with barbs or hooks on seeds

Facilitation

• Facilitation (+/+ or 0/+): an interaction where one species can have positive effects on another species without direct and intimate contact

  • For example, the black rush makes the soil more hospitable for other plant species

• Key distinction between three models is in the manner succession proceeds

  • Facilitation: species replacement is assisted by previous colonists
  • Inhibition: species replacement is prevented by previous colonists
  • Tolerance: species replacement is unaffected by previous colonists

• Other factors may also influence succession

Species Diversity

• Species diversity: the variety of organisms that make up the community
• It has two components: species richness and relative abundance
  • Species richness: the total number of different species in the community
  • Relative abundance: the proportion each species represents of the total individuals in the community
• Communities with higher diversity are
  • More productive and more stable in their productivity
  • Better able to withstand and recover from environmental stresses
  • More resistant to invasive species, organisms that become established outside their native range

Trophic Structure

• Trophic structure: the feeding relationships between organisms in a community
• It is a key factor in community dynamics
• Food chains link trophic levels from producers to top carnivores

Species with a Large Impact

• Dominant species: the species that are most abundant or have the highest biomass

  • Dominant species exert powerful control over the occurrence and distribution of other species

  • Sugar maples have a major impact on shading and soil nutrient availability in eastern North America 🡪affects the distribution of other plant species

  • Dominant species are most competitive in exploiting resources vs they are most successful at avoiding predators

    • Invasive species, typically introduced to a new environment by humans, often lack predators or disease

• Keystone species: exert strong control on a community by their ecological roles, or niches

  • They are not necessarily abundant in a community

Ecological Succession

• Ecological succession: the sequence of community and ecosystem changes after a disturbance
• Primary succession: when succession begins; occurs where no soil exists
• Secondary succession: begins in an area where soil remains
  • Early-arriving species and later-arriving species may be linked in one of three processes
  • Early arrivals 
    • May facilitate appearance of later species by making the environment favorable
    • May inhibit establishment of later species
    • May tolerate later species but have no impact on their establishment